This invention pertains to a method and apparatus for exciting or resonating resonant circuits, such as those which are commonly used today in so-called theft-protection merchandise tags. More particularly, the invention pertains to a method and apparatus whereby a single low-frequency oscillator may be used to excite or resonate an infinite number of different higher-frequency resonant circuits.
It is common practice today, in many fields of application, to use electrical resonant circuits as article or object identification tags. A very common use of these tags is made by merchants in the protection of their goods against shop-lifting. As an illustration, a tag containing a circuit resonant at a particular frequency is attached to an article. Usually there is provided a sensing station near the exits to a store, and in this station a transmitter transmits pulses at the frequency of the tags so as to excite any tag which passes through this region. Naturally, a tag passing through the region is excited or resonated by the transmission just mentioned, and in the intervals between transmitted pulses, the tag acts as a radiator itself of energy at the same frequency, which radiation is picked up by a suitable receiver that furnishes a warning signal.
There are many other applications where resonant circuits are similarly used. However, in the application just specifically described, as well as in the others, it is common to require a single controlled-frequency oscillator for each resonant circuit frequency which is used. Obviously, this can become an extremely costly proposition in situations where differentiation of objects or articles, as by using tags of different frequencies, is desired.
A general object of the present invention is to provide a unique method and apparatus which allows the use of virtually an infinite number of different-frequency resonant circuits, all excitable by a transmission source which uses but a single-frequency controlled oscillator.
Another object of the invention is to provide such a method and apparatus which are extremely simple and easy to use.
Various other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.